The invention relates to a process for mild hydrocracking hydrocarbon oils. More particularly, the invention relates to a mild hydrocracking catalytic process for treating vacuum gas oils and residuum hydrocarbon feedstocks.
In the refining of hydrocarbon oils, it is often desirable to subject the hydrocarbon oil to catalytic hydroprocessing. One such process is hydrocracking, a process wherein, in the typical instance, a gas oil or residuum feedstock is passed with hydrogen through a bed of catalyst active for cracking relatively high molecular weight compounds to more desirable, relatively low molecular weight compounds of lower boiling point. In addition, because the catalyst has hydrogenation activity, the cracked products are saturated by hydrogenation while organosulfur and organonitrogen compounds in the feed are converted to hydrogen sulfide and ammonia, respectively, both of which are usually removed in gas-liquid separators. Thus, the advantage of hydrocracking lies in the conversion of a sulfur-containing and/or nitrogen-containing gas oil feed, boiling, for example, mostly above about 700.degree. F., to a relatively sulfur and nitrogen-free product of boiling point below 700.degree. F., such as gasoline, jet fuel, diesel fuel, and mixtures thereof.
Recently, attention has been directed to "mild hydrocracking." The cost of constructing a hydrocracking unit operating at high pressures is quite significant and poses a major economic obstacle to its use. Accordingly, interest has developed in converting existing hydroprocessing units, such as hydrotreating or hydrodesulfurization units, into hydrocracking units. It is realized, of course, that hydrotreating units and the like, especially once-through units, are not normally designed for optimum hydrocracking conditions, and specifically, for the high pressures usually employed in commercial hydrocracking, i.e., above 1,500 p.s.i.g. Nevertheless, there is still an advantage if even some hydrocracking can be achieved under the low pressure constraints of typical hydrotreating or hydrodesulfurization units, and the challenge to the art is to discover hydrocracking catalysts having sufficient activity and activity maintenance to be commercially useful under such mild hydrocracking conditions.
Therefore, an aim of the art is to provide a mild hydrocracking catalyst having a high activity, selectivity and stability. Activity may be determined by comparing the temperature at which various catalysts must be utilized under otherwise constant mild hydrocracking conditions with the same feedstock so as to produce a given percentage (usually between 70 and 50 volume percent) of products boiling at or below 700.degree. F. The lower the temperature for a given catalyst, the more active such a catalyst is for mild hydrocracking. Alternatively, activity may be determined by comparing the percentages of products boiling at or below 700.degree. F. when various catalysts are utilized under otherwise constant mild hydrocracking conditions with the same feedstock. The higher the percentage of 700.degree. F.- minus product converted from the components in the feedstock boiling above 700.degree. F. for a given catalyst, the more active such a catalyst is in relation to a catalyst yielding a lower percentage of 700.degree. F.- minus product. Selectivity of a mild hydrocracking catalyst may be determined during the foregoing described activity test and is measured as that percentage fraction of the 700.degree. F.-minus product boiling in the range of middle distillate or midbarrel products, i.e., 300.degree. F.-700.degree. F. Stability is a measure of how well a catalyst maintains its activity over an extended time period when treating a given hydrocarbon feedstock under the conditions of the activity test. Stability is generally measured in terms of the change in temperature required per day to maintain a 40 volume percent or other given conversion (usually less than 50 volume percent).